Treatment of metal surfaces



United States Patent Int. Cl. C21d 1/46; (5215f 17/00; C235; 1/28 US. Cl. 20438 9 Claims ABSTRACT OF THE DISCLOSURE Metal parts are treated in a conventional salt bath comprising alkaline cyanides, cyanates and carbonates, to which alkaline ferriand ferro-cyanides are added and in which an increased carbonate content is maintained, after which the parts are coated with a tender metal. This treatment provides at the surface of the treated parts a hard micro-layer and over this layer, a porous layer. The salts of the bath penetrate these pores and dry therein after the treatment which, according to the components of the bath, procures a reserve of elements or radicals having exceptional lubricating qualities permitting the metal to be self-lubricating in most environments.

This invention relates to the surface treatment in molten salt baths of steel or composite pig-iron parts with a view to confer to the same a low friction coefficient in nonlubricating conditions.

The problem arises more and more frequently in industry of the friction of mechanical parts which cannot be lubricated because the mechanisms to which said parts belong must operate in unusual environment-s. Thus mechanisms should be capable of operating in the nuclear industry, in such gases as CO and in the chemical industry in media such as water, heavy water or acids, while space navigation requires assemblies capable of a frictional operation in vacuo.

It is furthermore sought to obtain operation of such mechanisms with a very low amount of energy. For instance, the movement of space navigation mechanism-s should need such a restricted energy consumption that the value of the friction coetficient becomes a primordial variable.

Surface treatments in molten salt baths are known which make it possible to obtain mechanical parts adapted for non-lubricated, reciprocal friction in relatively good conditions. The baths used for such treatments mainly comprise alkaline cyanides, cyanates and carbonates to which sulphur ions may or may not be added. Baths of this type are found on the market under the trade names Sulf-Inuz (S.A.T.S. and Ateliers Partiot Cementation, France; French Patent Nos. 942,387, 1,057,237, 1,073,- 922, 1,058,076) and Tenifer ('Degussa, Germany; French Patent Nos. 1,198,242, 1,230,471 and 1,255,191). Such baths result among others in the formation on the surface of the parts treated of a coating of a compact mixture of carbides and nitrides, commonly called a hard micro-layer. Said layer is compact and has an irregular and pricked outer aspect. It should be noted here that all such baths reach after a few days of use, a state of dynamic equilibrium which results among others in the conversion of cyanides to cyan-ates and of cyanates to carbonates, and in the precipitation at the bottom of the crucible, of slurries which are a mixture of alkaline carbonates and of iron carbonates originating from the crucible, generally made of steel, and from the treated parts. When the bath is properly maintained first by a daily elimination of the slurries and second by a corresponding addition of fresh cyanides and cyanates, there results an equilibrium content-of carbonates, specific to the known bath considered;

3,507,757 Patented Apr. 21, 1970 thus, for example, the content of alkaline carbonates corresponding to said equilibrium in the bath commercially known as Sulf-Inuz is comprised between 30 and 35% by weight of the bath.

The treatments by means of the above mentioned salt baths show however the following drawbacks:

the friction coefiicients are relatively high, of the order the surface conditions after treatment are imperfect, and

the resistance to corrosion remains very insufi'lcient.

On another hand it is known, according to the theory of compatibility of metals that the use of friction couples of insoluble structure such as, for example, aluminiumcadmium or copper-molybdenum, should also lead to relatively good performances. In the two above mentioned couples one of the metals in each case may be deposited electrolytically as a thin layer: cadmium in the first couple, copper in the second. Such electrolytic metal coatings, however, have only seldom been used industrially in the' field of friction, since the present knowledge does not provide coatings which adhere sufiiciently firmly to the underlying metal (see J. J. Caubet, Thorie et Pratique Inrllu-ztrielle du Frottement, E. Dunod Technip., Paris,

The lubricating qualities of alkaline or alkaline-earth salts comprising at least one of the atoms or radicals S, Se, Te, F, Cl, Br, I, P, CN, CO are also known, as shown by experiments made with the convnetional Faville- Levally machine, which is normally used for the test of lubricants and comprises two V-shaped jaws with the sides of the Vs diverging to form a right angle. These jaws are made of hardened semi-hard steel and are arranged to permit symmetrical clamping under a predetermined adjustable pressure on a cylindrical test piece connected to a rotary driving member by means of a pin of 2 mm. diameter extending diametrically through an end of the test piece. The jaws are progressively tightened while the test piece rotates about its axis. Either the test piece jams between the jaws, or when the couple formed by the test piece and jaws is particularly adapted to friction, the test piece heats, flows and is not forged, the jaws forming a section of restricted diameter on the test piece.

When a test piece made of semi-hard, XC32) steel, 6.5 mm. in diameter and 40 mm. long is rotated at 350 r.p.m. between jaws made of the same steel, jamming takes place within a few seconds under a charge of the order of 1,000 to 1,500 newtons. On another hand when an identical test piece has been previously immersed in a tepid, aqueous solution of a mixture of salts as above mentioned, there remains on the test piece after drying a thin, whitish salt coating, and the test piece thus treated is capable of fantastic performances, under charges of millions of newtons 5, as disclosed in French Patent No. 1,425,430.

Finally I. V. Kragelski in Friction and Wear London, Butterworths, 1965 has disclosed a theory according to which friction takes place in conditions which are the best when there exists a high positive gradient of the mechanical characteristics of the material from the outer surface towards the inside of the material, in a direction perpendicular to the friction surface. K. Ragelski cites as examples of surfaces having a high positive gradient favoring friction, either a very thin superficial layer of oxides or sulfides adsorbed on a very hard sub-layer, or a very thin coating of a soft metal on a harder base.

It has however now been discovered that much better results can be obtained by means of a treatment such that, contrary to the rule established by Kragelski, on one hand the gradient of the mechanical characteristics when penetrating from the outer surface in the inner part of the treated material is first positive, passes through a zero value at a given depth, and is negative thereafter, which shows that there exists at an optimum depth a maximum of the mechanical characteristics, and further, that the area of maximum mechanical characteristics also shows a high ductility, that is to say simultaneously a high resilience, a high elongation and a high striction. Experience has shown that in the case of conventional mechanical construction the optimum depth where the maximum hardness and ductility should be reached is somewhere between and 20 microns.

One object'of the present invention is to provide a method for surface treatment of steel or composite pig-iron parts in molten salt baths, providing a substantial lowering of the friction coefficient, and consequently of the wearing of the treated parts and of the parts cooperating therewith.

Another object is to provide means for obtaining at the surface of steel or composite pig-iron parts, the hereinabove mentioned, desired gradient of mechanical characteristics, first positive, then of zero value, and finally negative.

Still another object is to provide a method for surface treatment of said parts in molten salt baths in which an outer, porous layer is formed on the conventional compact, hard surface, on which porous layer a surface coating of a judiciously chosen tender metal may adhere with extraordinary strength.

According to the invention, the steel or composite pigiron parts are treated during a period of time of the order of at least 2 to 4 hours and at a temperature of the order of 550 to 600 C. in a conventional molten salt bath comprising alkaline cyanides and cyanates and at least an alkaline carbonate, said bath having added thereto, per weight of the total bath, from 1 to 8 parts per thousand of alkaline ferri-cyanide and 0.5 to 2 parts per thousand of alkaline ferro-cyanide, while maintaining the content of alkaline carbonates in the bath at a value from 1 to 7% above the equilibrium content thereof in the initial bath, the parts under treatment being electrically insulated while the bath is connected to ground after which the treated parts are extracted from the said bath and coated with a thin layer of tender metal.

The preferred additions of alkaline ferriand ferrocyanides are of 3 to 7 parts per thousand for the ferricyanide and about one part per thousand for ferro-cyanide. When the bath according to the invention is used continuously the desired contents of ferriand ferro-cyanides should be maintained by adding the above mentioned proportions of said compounds from time to time, at most every 5 hours, and at least every 40 hours.

Maintaining of the desired content of carbonates in the bath may be obtained by the simple addition of ferriand ferro-cyanides when the additions are sufficiently on the high side of the above-mentioned ranges for each compound. It may also be obtained by addition of the required amount of carbonate, jointly with the addition of cyanides which may then be added in smaller quantities, but always within the disclosed ranges. The most advantageous content of alkaline carbonate is about 36 to 37% by weight of the total bath.

The conventional baths to which alkaline ferriand ferro-cyanides are added and the content of alkaline carbonates of which is increased according to the invention may or may not contain sulphur. Said baths may also comprise compounds supplying the elements Se, Te, F, Cl etc. as above listed.

It has been found that the above disclosed salt bath treatment leads to the formation at the surface of the treated parts, first a hard micro-layer obtained with conventional baths, then, over said layer, an outer, porous layer, the very fine pores of which have a glove finger shape and open outside. As a result the salts in the bath penetrate said pores and dry therein after the treatment, which according to the components of the bath procures a reserve of elements or radicals such as S, Se, Te, F,

, Cl, Br, I, P, CN, CO theexceptional lubricating qualities of which have been hereinabove noted.

The electrical insulation of the parts while being treated from their support results in obtaining a better aspect of the treated parts, the porous layer being much more regular in thickness.

Once the salt baths treatment has been carried out the parts, after being preferably pickled and de-greased, and possibly passivated, are coated with a thin layer of tender metal.

As considered herein, a tender metal is a metal such as cadmium, copper, silver, gold, lead, and the like having a good resistance to atmospheric corrosion, a hardness lower than HV, and which are capable of being deposited by means of a chemical bath or electrolytically. the choice of the tender metal used depends on the metal which forms the part with which the part treated according to the invention is intended to cooperate frictionally; the selected metal should be that which presents with the metal of the coacting part the best binary diagram of insolubility. Thus, for example, the selected metal should be:

cadmium when the treated part is intended to operate frictionally with iron alloys or light alloys or metals,

copper or gold with a chromium or molybdenum coacting surface,

silver with a stainless steel coacting surface, or

lead when the treated part is intended to cooperate with a ferrous alloy in a reducing atmosphere.

The coating with the layer of tender metal should be carried out by chemical or electrolytical process, excluding any physical process such as gun spraying or the like. The thickness of the tender metal coating may be of the order of 1 to 10 microns.

The porosity of the layer on which the metal coating is applied provides the possibility for said coating to adhere remarkably strongly, contrary to what happened before when it has been tried to provided parts treated in conventional salt baths with such a metal coating without following the teachings of the present invention.

When the pores on the underlying layer are sufficiently large the metal penetrates therein which still further reinforces the adherence. Where the pores are insufliciently large the tender metal coating forms on each of said pores a kind of well through which said pores communicate with the outside, thus enabling the dried lubricating salts therein to exude regularly thereafter at the surface of the coating when the treated part is submitted to friction.

The parts treated according to the invention present in the absence of lubricant-whether in air or water, or aqueous solutions, in vacuo or in a gaseous environment such as cO -frictional performances which are vastly superior to anything disclosed up to date, their friction coefficient being exceptionally low.

Said parts may be used for establishing friction couples already theoretically known for their excellency but which could not be obtained in practice with sufficient security by means of any known process; such is the case for the well-known aluminium-cadmium friction couple which has hitherto practically never been used because it was impossible to obtain cadmium coatings sufiiciently adherent to the underlying metal, and because, on another hand, cadmium cannot be used as a mass in mechanical construction because of its insufficient mechanical properties. Now the parts treated according to the invention, when the tender metal selected for the coating is precisely cadmium may cooperate frictionally with aluminium alloys with performances unknown up to date and friction coefficients of the order of 0.08. Furthermore the resistance of such parts to corrosion is exactly that of a conventional cadmium coated part, and thus much higher than that of parts treated in a conventional salt bath.

The above mentioned rule of obtaining a gradient of mechanical properties which first is positive, then passes through zero, and is finally negative is completely verified in the surface of the parts treatedaccording to the invention, the maximum value of the mechanical properties being found at a depth of 5 to 30 microns under the outer surface.

The following are two non-limitative examples of practical working of the invention.

EXAMPLE I A salt bath having the following composition was melted in a conductive crucible at 570 C.:

Percent To said bath were added, by weight of the total bath, 5% carbonate, 3% potassium ferri-cyanide and 1% potassium ferro-cyanide.

A cylindrical test piece of XC35f steel with a perliteferritic structure, of 6.5 mm. diameter and 40 mm. length was immersed in the bath during 3 hours, the temperature being maintained at 570 C. and said test piece being completely insulated electrically from its support whereas the crucible was grounded.

The test piece thus treated showed, down to a depth of a few tensv of mm. from the surface of an interstitial penetration of nitrogen increasing its hardness and its flowing aptitude, and at the surface, ,an inner, hard, compact microlayer about 10 microns thick, insinuating itself at the joints of the metal grains, said micro-layer being covered with an outer, porous layer about 15 microns thick, the pores of which were in the form of glove fingers opening at the outside and generally having an increasing width towards the outer surface.

The test piece was then submitted cessive treatments.

(a) Washing during 2 hours in flowing water.

(b) Brushing with a nylon brush.

(c) Drying in a drying cupboard.

(d) Pickling by immersion during 35 minutes at room temperature in a bath of the following composition:

to the following suc- Hydrochloric acid, by volume /s Water, by volume /5 Gelatin, mg. per liter 50 (e) Washing and rinsing in tap water. (f) Electrolytic degreasing in an aqueous bath containing, in gr. per liter:

Trisodium phosphate 40 Sodium metasilicate 40 Sodium carbonate 40 Lauric alcohol sulphate 20 Complexing agent 1 used in anodic phase Treatment temperature-60-70 C.

Treatment period34 minutes.

Graphite anodesSurface five times greater than treated parts.

Current densityl-15 a. per sq. dm.

(g) Washing and rinsing in tap water.

(h) Chemical attack during 45 seconds in a aqueous nitric acid solution.

(i) Rinsing in flowing water.

After said sequence of treatments the test piece received electrolytically a cadmium coating about 8 microns thick in an aqueous bath at 20 C. containing, per litre:

Gr. Cadmium cyanide 58.5 Sodium cyanide 110.5 Sodium hydroxide 80.0

The treated test piece was mounted as cathode (current density 1.5 a. per sq. dm.) and the anodes were of pure cadmium and operated with a current density of 0.75 a. per sq. dm.

The test piece was then finally rinsed in flowing water, and dried.

The microscopic examination of said test piece showed that cadmium had filled the pores of the superficial porous layer in the layer section of said pores, thus procuring a remarkable adherence of the cadmium layer at the surface of the test piece, the proportion of cadmium decreasing from the outside toward the inside. Under the porous layer thus partially filled with cadmium appears the compact, hard micro-layer representing, at a depth of the order of 20 microns, the locus of the maximum mechanical properties, followed by the area of the interstitial nitrogen solution of decreasing nitrogen contents, which-is the area in which the gradient of mechanical properties is negative.

The test piece was submitted to a test in the hereinabove described Faville-Levally machine, provided with annealed and rectified XC35f steel jaws submitted to a load of 3000 newtons.

The friction coeflicient in air was only 0.08 before the parts began heating and evolved particularly favorably with time, as well in air as in water. By way of comparison the friction coefiicient of a part having a conventional, untreated steel provided with a cadmium coating of the same thickness, shows in the same conditions a friction coefficient of the order of 0.16 to 0.18 the cadmium layer separating progressively from the steel surface in the course of friction wear, whereas in the case of the test piece treated according to the invention there is no separation of the cadmium layer which remains firmly adherent to the surface.

1 Furthermore the friction conditions with an aluminium counterpart were excellent.

EXAMPLE II The following bath was melted in a conductive electrically grounded crucible at a temperature of 560 C.:

Percent by weight To said bath was added with 7% potassium ferri-cyanide andh 1% potassium ferro-cyanide, by weight of the total 3SNCD6 steel test piece, tempered and annealed at 600 C. was immersed in said bath while being completely electrically insulated from its support. After 3 hours of treatment followed by cleaning and brushing, the microscopic'study of the cross section of the test piece showed the existence of a very porous outer micro-layer, about 10 microns thick, then a sublying, hard compact micro-layer of about 8 to 10 microns, the hardness under said layer decreasing until the initial hardness of the test piece is reached at a depth of about 0.5 mm.

Said test piece was submitted to the same treatment as described at (a) to (j) in Example I, followed by an alkaline rinsing in an aqueous 3% solution of sodium cyanide, then by a further rinsing in water.

The test piece was then electrolytically copper coated in the following aqueous bath at 60 C., the current density at the cathode being 3 a. per sq. dm. and that at the cathode 2 a. per sq. dm.:

Finally the test piece thus provided with a copper coating of about 3 microns thickness was rinsed in cold water, then in hot water and dried.

The test piece thus treated proved to be capable of hitherto unattained performances when caused to operate frictionally with a molybdenum counterpart, the friction coefficient being as low as 0.08 and the wear being five times less than in the case of friction on the same molybdenum surface of a similar test piece simply copper coated without having undergone the complete treatment according to the invention.

When the bath of this example was used continuously for the industrial treatment of steel parts, the required conditions to to ferriand ferro-cyanide and alkaline carbonate contents were maintained by proceeding each morning with the same addition of 7% ferri-cyanide and 1% ferro-cyanide.

What we claim is:

1. A method for the treatment of steel and composite pig-iron parts in a molten salt bath comprising alkaline cyanides, cyanates and at least one alkaline carbonate, for a period of at least 2 to 4 hours at a temperature of 550-600" C., comprising adding to said bath, by weight, 1 to 8 parts per thousand of alkaline ferri-cyanide and 0.5 to 2 parts per thousand of alkaline ferro-cyanide, while maintaining the content of alkaline carbonates in the bath at a value of from 1 to 7% above the equilibrium content thereof in the initial bath, the parts. under treatment being electrically insulated while the bath is connected to ground, after which the treated parts are extracted from the said bath and coated with a thin layer of tender metal.

2. A method as claimed in claim 1, in which the addi- 3. A method as claimed in claim 1, in which, in case of continuous use of the bath the addition of ferriand ferro-cyanides is operated from time to time at most every 5 hours and at least every 40 hours.

4. A method as claimed in claim 1, in which the content of alkaline carbonates in the bath is maintained at a value of from about 36 to 37% by weight.

5. A method as claimed in claim 1, in which the tender metal used for finally coating the treated part is a metal selected from the group consisting of cadmium, copper, silver, gold and lead.

6. Steel and composite pig-iron parts treated according to claim 1, and having a superficial zone consisting from the inside to the outer surface, of an area in which nitrogen is interstitially dissolved in the metal, the nitrogen contents decreasing in the direction of the surface, a hard, compact micro-layer, a porous microlayer having pores opening towards the outer surface and a coating of tender metal strongly adherent to said micro-layer.

7. The method according to claim 5, in which the tender metal is deposited by means of a chemical bath.

8. The method according to claim 5, in which the tender metal is deposited electrolytically.

9. The method according to claim 1, in which the tender metal coating has a thickness of from 1 to 10 microns.

References Cited UNITED STATES PATENTS JOHN H. MACK, Primary Examiner W. VAN SISE, Assistant Examiner US. Cl. X.R. 

